It has long been known that low-field MRI produces high contrast images, but the weak field leads to a low signal to noise (S/N) ratio at poor resolution. Increasing the field increases the S/N ratio and, therefore, the resolution but decreases the contrast, so that high-field images have high resolution but poor contrast. There have been many attempts to overcome this limitation and to provide high-contrast high-resolution MRI images.
U.S. Pat. No. 5,168,226A to Hinks discloses a method whereby the total scan time may be shortened without losing resolution. The method disclosed in U.S. Pat. No. 516,226A comprises executing a fast spin echo pulse sequence in which a plurality of views are acquired and the fast spin echo pulse sequence is employed to acquire views from a plurality of separate images during a scan. The low-order phase encoding views are acquired for each image and stored in separate image data arrays, whereas the high-order phase encoding views are acquired only once and stored in all of the image data arrays. Each image data array is employed to reconstruct a separate image using standard reconstruction methods and apparatus. The desired T2 contrast is produced primarily by the low-order views of each image and the high-order views enhance the structural details of each image. Accordingly, only the low-order views need be acquired separately for each image to provide the desired T2 contrast, and a single set of high order phase encoding views can be used to fill in the structure details in all of the images. However, this method provides a relatively small enhancement of the contrast unless a large number of high-order phase encoding views are acquired.
Another method of improving contrast is by adding contrast agents to the region of interest, such as administration of a paramagnetic contrast agent (for example, gadolinium) to blood vessels and creating the MRI images at a time when the concentration of contrast agent is at a maximum. This method is disclosed in U.S. Pat. No. 5,479,925A to Dumoulin et al., among others. The method is adapted to enhancing contrast in medical MRI but has limited utility in industrial application where there are no obvious sub-domains (such as blood vessels in medical MRI) into which to introduce the contrast agents and where the presence of a contrast agent in a finished product may well be undesirable.
US Patent Application US2004169512A to Jara discloses a method of combining three image-post-processing phases for the purpose of generating high-quality quantitative MR images (proton density (PD), T1, and T2) as well as high-quality virtual MR images with continuously adjustable computer-synthesized contrast weightings, from source images acquired directly with an MRI scanner. Each of the image-post-processing phases uses one or several new computer algorithms that improve image quality with respect to prior art, including linear-combination-of source-images (LCSI) algorithms for generating PD images and model-conforming algorithms for generating Q-MR images of tissue properties that influence NMR relaxation. However, the method depends on the presence of materials with different relaxation times in different parts of the scan (such as white matter and cerebrospinal fluid) to enable the enhanced contrast.
It is therefore a long felt need to provide an MRI device which provides high-contrast and high-resolution images especially for detecting contaminating or hazardous particles.